DOS 514 - Week 3 Discussion
Initial Post: Protons are a Fantastic, But Specialized Tool
Protons and neutrons are the two basic particles that make up the nuclei of atoms.1 They are both very stable, but protons are the most stable particles, with half-lives presumed to be more than 10^32 years, which is many orders of magnitude longer than the current age of the universe. Protons have a unit positive charge, and the number of protons in a nucleus is what determines both the total charge of that nucleus and what kind of element that nucleus is. A solitary proton is the nucleus of a hydrogen atom.
In the context of radiation therapy, a proton is considered to be a heavy particle, with a mass almost 2000 times that of an electron.1 Other heavy particles such as alpha particles, carbon ions, and even heavier atomic nuclei are all several times heavier than protons, but they remain in the same order of magnitude, whereas the entire group is several orders of magnitude heavier than electrons. Neutrons are also heavy particles, but since they are not charged particles, they behave with their own set of rules. Positively charged heavy particles such as protons follow a distinct pattern of behavior when traveling through a medium. According to Khan, the rate of energy loss of a charged particle traveling through a medium is proportional to the square of its charge and inversely proportional to the square of its velocity. Since the charge of protons remains constant, the velocity is the key component in this behavior.
Since protons mass considerable mass, especially when moving at relativistic speeds, they are not readily scattered in lateral directions like electrons. They mainly transfer their energy to matter through inelastic collisions with electrons, which transfer some of the protons' kinetic energy to the electrons, either exciting them or stripping them away from their parent atoms, ionizing them. Protons can also be elastically scattered by near passes with atomic nuclei, but the scatter angle is usually very small, and these interactions do not rob kinetic energy. Direct inelastic hits on nuclei are rare. Elastic scatterings allow protons to interact with many electrons along their path, giving up a small amount of energy and velocity with each interaction. As the velocity decreases, the amount of energy lost in each interaction increases, and eventually the proton will be going slow enough that it will capture an electron, dumping its remaining energy and forming a full hydrogen atom. After that, there will be no further interactions with atoms along the path direction. This behavior of low, then increasing, then sharply increasing, then zero energy transfer creates a depth-dose curve shape called a Bragg peak.
The pattern of scattering for a given material is very predictable, based on its stopping power, which is a quantification of energy loss per unit of length. Low-Z materials have higher stopping power than high-Z materials, but low-Z materials cause much less angular scatter than high-Z material. Since living tissue is primarily composed of low-Z atoms like hydrogen, carbon, oxygen, and nitrogen, proton beams used for therapy typically have very little lateral scatter and highly predictable penetration depth based on entrance energy.
This produces several very desirable traits for radiation therapy:
- dose absorption in the early part of the path is relatively low
- maximum energy transfer occurs at the targeted range
- depth of maximum energy transfer is predictable
- there is no energy transfer beyond the target
- lateral scatter of the beam is minimal, allowing tightly sculpted dose delivery
When planning radiation therapy with heavy particle, planners will want to place the Bragg peak inside their target so that the target receives the full hit at the end of range, tissue in front of the target will receive moderate dose, and distal tissues will receive no dose. This allows beam angles that point directly towards distal critical avoidance structures, as long as the planner is careful about having the beam stop before it hits the critical structure.
Since the Bragg peak occurs a very narrow depth range, planners will usually shoot particles with several closely spaced decreasing energies such that the Bragg peak of each energy occurs a few millimeters behind the last, stepping continuously closer back to the entrance point. Since there is already some dose deposition from higher energy particles that have already passed through a given point, each successive lower energy layer only has to be delivered for a relatively short period of time to boost the selected layer to the prescription dose. This stacking of the Bragg peaks of several closely spaced energies results in a relatively flat dose profile across the tumor inside a "spread out Bragg peak" (SOBP).
In my experience at the SCCA Proton Therapy Center, proton treatment has been extremely valuable for pediatric cases, where radiation dose to uninvolved tissue must be minimized; for neurological cases, where univolved parts of the brain can be spared; and in retreatments near critical structures, where we can select beam paths that have not have been used previously because they point towards the critical structures.
Protons are not ideal for all therapies, and many types of cases are in fact better treated through other forms of therapy. Protons will not replace photon treatment, but they are an excellent tool to have available for those cases where the unique properties of protons can benefit the patient.
- Khan FM, Gibbons JP. The physics of radiation therapy. 5e. Philadelphia, PA: Lippincott Williams & Wilkins; 2014.
- Spread out Bragg peak. http://jicru.oxfordjournals.org/content/7/2/11/F4.large.jpg. Journal of the ICRU Website. Accessed on September 17, 2014.